Image manipulation in obj-c / iphone

colourise.m

typedef unsigned char byte;
#define Clamp255(a) (a>255 ? 255 : a)

+ (UIImage*) fromImage:(UIImage*)source toColourR:(int)colR g:(int)colG b:(int)colB {

    // Thanks: http://brandontreb.com/image-manipulation-retrieving-and-updating-pixel-values-for-a-uiimage/
    CGContextRef ctx;
    CGImageRef imageRef = [source CGImage];
    NSUInteger width = CGImageGetWidth(imageRef);
    NSUInteger height = CGImageGetHeight(imageRef);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    byte *rawData = malloc(height * width * 4);
    NSUInteger bytesPerPixel = 4;
    NSUInteger bytesPerRow = bytesPerPixel * width;
    NSUInteger bitsPerComponent = 8;
    CGContextRef context = CGBitmapContextCreate(rawData, width, height,
                                                 bitsPerComponent, bytesPerRow, colorSpace,
                                                 kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
    
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
    CGContextRelease(context);

    int byteIndex = 0;
    for (int ii = 0 ; ii < width * height ; ++ii)
    {
        int grey = (rawData[byteIndex] + rawData[byteIndex+1] + rawData[byteIndex+2]) / 3;
        
        rawData[byteIndex] = Clamp255(colR*grey/256);
        rawData[byteIndex+1] = Clamp255(colG*grey/256);
        rawData[byteIndex+2] = Clamp255(colB*grey/256);
        
        byteIndex += 4;
    }
    
    ctx = CGBitmapContextCreate(rawData,
                                CGImageGetWidth( imageRef ),
                                CGImageGetHeight( imageRef ),
                                8,
                                bytesPerRow,
                                colorSpace,
                                kCGImageAlphaPremultipliedLast ); 
    CGColorSpaceRelease(colorSpace);
    
    imageRef = CGBitmapContextCreateImage (ctx);
    UIImage* rawImage = [UIImage imageWithCGImage:imageRef];  
    CGImageRelease(imageRef);
    
    CGContextRelease(ctx);  
    free(rawData);

    return rawImage;
}